1. Field of the Invention
This invention relates to a method and apparatus for detection of fault conditions and, more particularly, to a method and apparatus for detecting radio frequency signals in a turbine generator which discriminates between false fault signals (for example, noise signals) and real fault signals (for example, signals generated by arcing).
2. Description of the Related Art
A turbine generator generally has six leads. For example, a three-phase generator has three "hot" phase leads and three neutral leads connected to ground. A single neutral lead is thus common to all three phases A conventional radio frequency monitor (RFM) senses all RF signals from the neutral lead. If a specified threshold level is sensed as a result of an RF signal, the RFM issues an alarm.
RF signals can occur as a result of at least three conditions. Very short duration pulses of low voltage or current occur due to partial discharges within the generator and isophase ducts. These short duration pulses occur more or less at a constant rate, and are not indicative of a problem within the generator i.e., they are "false" signals. However, conventional RFM's will sense the short duration pulses and add their contribution to the RFM level, falsely indicating a generator malfunction. This is one type of false signal.
A second false signal occurs due to external electrical switching circuits containing SCR's, Triacs, etc. Signals generated from the switching circuits are typically very large spikes (high voltage or current) which may or may not be synchronous. They usually occur intermittently, but with conventional RFM's they are easily confused with, and interpreted as, an arcing event. These false signals may also incorrectly contribute to an RFM alarm.
Real fault signals are generated by arcs. These arcs may be internal or external to the generator and typically are associated with broken winding strands and dirty or misadjusted shaft grounding brushes. Sensing of an arc indicates a possible problem in the generator system.
Conventional RFM's cannot discriminate between the false signals and the real signals, resulting in false alarms and a reduced ability to detect arcing events.